Add assistant response copy action with copy format setting

[leonardoxr]

Summary

• add a dedicated copy action for completed assistant responses in the chat timeline
• add a client-only assistant response copy format setting with markdown and plain-text modes
• keep code-block copy behavior unchanged
• add markdown-to-plain-text serialization and regression coverage

Validation

• bun fmt
• bun lint
• bun typecheck
• bun run test -- src/lib/assistantMessageCopy.test.ts src/components/chat/MessagesTimeline.test.tsx in apps/web
• bun run test:browser -- src/components/ChatView.browser.tsx -t "copies the raw assistant markdown by default|copies assistant responses as plain text when the setting is enabled|keeps markdown code-block copy scoped to the code block" in apps/web

Assets

Chat before

Chat after

Settings before

Settings after

Interaction details

Issue: #1455

Closes #1455

---

Note

Medium Risk
Adds new clipboard/copy behavior and markdown-to-plain-text transformation in the chat timeline, and adjusts virtualization height estimation based on that UI. Also updates DrainableWorker transactional primitives, which could impact async queue/drain behavior if the new Effect APIs behave differently.

Overview
Adds a dedicated “Copy response” button for completed assistant messages in the chat timeline, with the copied content derived from a new assistantResponseCopyFormat client setting.

Introduces assistantMessageCopy utilities to resolve assistant copy text as raw markdown or a deterministic plain-text rendering (via unified/remark-* with LRU caching), updates timeline virtualization height estimation to account for the extra action row, and extends settings UI + legacy localStorage migration to support the new preference.

Refactors MessageCopyButton to support dynamic text and customizable labels, expands unit/browser coverage for copy behavior and height estimation, and updates DrainableWorker to use newer Effect transaction/retry APIs.

^{Written by Cursor Bugbot for commit 314644c. This will update automatically on new commits. Configure here.}

Note

Add copy action for assistant responses with markdown or plain-text format setting

• Adds a hover-visible 'Copy response' button to assistant message rows in MessagesTimeline, copying content as raw markdown or rendered plain text based on a user setting.
• Adds assistantResponseCopyFormat to ClientSettingsSchema (defaulting to 'markdown') and a settings UI in the General panel to switch between 'Raw markdown' and 'Rendered plain text'.
• Implements markdownToPlainText in assistantMessageCopy.ts using unified + remark-parse + remark-gfm with an LRU cache to avoid repeated parsing.
• The copy button is hidden during streaming and when plain-text resolution produces an empty result; row height estimation in timelineHeight.ts adjusts accordingly.
• MessageCopyButton is extended to accept a lazy text resolver function and updates its label/aria state to reflect copied status.

^{Macroscope summarized 314644c.}

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[leonardoxr]

@juliusmarminge
Hello, there! first pr ever on a public repo.

I was pondering if i should've skipped the setting, to make it smaller. but I'm hoping for feedback.